Full color film must sense in three color dimensions. These three dimensions are sensed by three monochrome, or black and white emulsions, each acting as a sensor for a different spectral sensitivity, or color. In digital imaging terminology, each of these emulsions produces a separate color channel, or component of the full color image. Historically, there have been several topological arrangements of these sensors. The first color film, Duffycolor, used colored rice grains to create a red, green, and blue matrix over black and white film like a modern CCD matrix. Polaroid made an instant transparency film that lay red, green, and blue stripes over a black and white film, similar to the shadowmask in a color CRT. The original Technicolor process exposed three spatially separate color images on black and white film, created a separate dye transfer matrix from each, and dyed a single layer of receiving film with three dyes from the three matrices to make the theatrical print. It wasn""t until the advent of Eastman""s multilayer Kodachrome that a color film suitable for the mass market became practical.
Today virtually all color film includes multiple layers stacked on top of each other Light impinging on the film passes through all the layers. The layers have different spectral sensitivities, so depending on the color of the light, it will expose a specific layer. In most films, each layer also is given at manufacture a unique color coupler, or piece of a dye molecule, that will react with byproducts of development to form a full dye appropriate to the color sensitivity of that layer. After development, the silver image is bleached away, leaving a color image composed of dyes in layers. If you abrade a color film, black areas will first turn blue as the yellow layer is removed, then cyan, as the magenta layer is removed, and finally white as all layers are removed.
Kodachrome has multiple layers, but uses a unique process limited to reversal transparency film that does not require couplers to be stored in the undeveloped film. After a first development to expend to silver the exposed silver halide, the unexposed halide is flash exposed and processed in a developer containing its own color coupler. In Kodachrome the flash exposure is done one color at a time to flash one color layer at a time, followed after each flash by a developer with a coupler specific to the color sensitivity of that layer. Kodachrome development is very difficult, and only a few labs in the world process Kodachrome. However, by eliminating color couplers from the film during exposure, light scattering in the emulsion is reduced, giving Kodachrome an extra measure of clarity. Negative film gives a much wider latitude than reversal film, but in the prior art the Kodachrome process was limited to reversal film, and there was no way to obtain the latitude advantage of a negative film and at the same time the sharpness advantage of a film without couplers.
Prior art color film was limited to operate in the RGB color space because each layer had to map to a specific dye that would develop an image, or color channel, that could be viewed or printed directly without color space conversion. Thus, a red sensitive layer was needed to generate cyan dye to modulate the amount of red light passing through the developed film, a green sensitive layer generated magenta dye, and a blue sensitive layer generated yellow dye. This traditional requirement of direct viewability of the chemically developed image placed a restriction on prior art color film to sense light in the RGB color space. Furthermore for pure colors to be recorded and viewed without color space conversion, the layers had to sense relatively pure red, green, and blue, without cross, contamination of colors. For example, if the magenta forming layer sensed blue light in addition to green, and the red sensitive layer sensed blue in addition to red, then a blue flower would expose not only the blue layer, but also the green and red, forming a shade of gray when developed conventionally and viewed directly as a color image.
Further, the depth ordering of sensitive layers of conventional color film was limited by the universal sensitivity of silver halide to blue light. Silver halide always is blue sensitive. In addition to this blue sensitivity, dyes may be added to trap the photons of other colors and couple them into the halide crystals. Thus a green sensitive layer is actually sensitive to both blue and green, and a red sensitive layer is sensitive to both blue and red. Green only and red only sensitive layers, which are needed for direct control of the magenta and cyan dyes, can only be realized by filtering out blue light with a yellow filter. In color film this is accomplished by adding a yellow filtering layer. This yellow layer must of course, be placed above the red and green sensitive layers in order to filter blue light from those layers, and must be under the blue sensitive layer so as not to occlude blue light from that blue sensing layer. Thus in the prior art, the blue sensitive layer had to be on top, over the yellow filter and therefore over the red and green sensitive layers.
Undeveloped silver halide scatters light with its milky consistency. When held up to the light, undeveloped film acts as a diffuser and attenuator. This can be observed while loading film into a camera. Each sensitive layer in color film degrades the image for lower layers, both by diffusing and thus blurring the light, and also by using, reflecting, and absorbing some of the light, thus dimming the light to lower layers, and requiring those layers to be more sensitive, and hence grainier. Only the top layer receives the full unattenuated, unblurred light.
Because the human eye senses detail almost totally in the luminance, ideally the full luminance should be sensed in that top layer. Unfortunately only one layer can be on top. The next choice would be to make that one layer the green sensor because green is responsible for over half the luminance. But as we have just shown, the blue layer must be on top, followed by a yellow filtering layer prior to the green. Blue is responsible for only about 10% of luminance, therefore the mandatory requirement that blue be on top means that almost all the luminance is sensed at lower layers where the image is dimmed and blurred. Most of the advancement in film technologies has been in color film, and yet today a fine-art black and white print has a clarity and vivaciousness that is not matched by prior art color film.
There have been historic and niche attempts to place green or red on top. Most interesting is color print paper which places blue at the bottom and red on top. The immediate question is how the red sensing layer is shielded from blue. Actually it isn""t shielded, and in fact the red sensing layer is nearly as sensitive to blue light as to red. Several conditions unique to printing paper make this practical. First, because the high contrast paper views a low contrast negative, the density range of exposure needed to go from white to black is only about 10:1, as opposed to camera film that must respond over a 1000:1 range, and a separation of 100:1 between red and blue is therefore adequate for print paper. Printing paper sees light that typically emanates from an orangish incandescent light, filtered by an orangish filter pack that removes typically 80% of the blue from the lamp, and is then focused through a negative that has the base orange cast of the coupler mask. The deep orange of the resulting light takes the place of the yellow filter level in camera film for the low contrast image of negative printing, and the blue sensitive layer is made about 100 times more sensitive than the red layer to compensate for the orange light, which is possible because of the relatively low base sensitivity of printing paper compared to camera film.
The approach used in printing paper to put the red on top would not work with camera film because, first, the film must respond over a range of 1000:1, not 10:1, second, the light coming through the lens is not deep orange, and third, even if it was, to match an ASA 400 red sensitivity, a blue layer 100 times more sensitive would need to be ASA 40,000, which would be very grainy. Nevertheless there have been attempts to place the green layer on top. Despite the improvements mentioned earlier, the unavoidable color muting caused by crosscolor contaminations caused these attempts to fail to be accepted by the market.
Although prior art color film senses with Red-Green-Blue channels, directly capturing three dimensional RGB color space, in image processing and storage other color spaces are possible that better exploit the needs of human vision. A common color space includes luminance and chrominance. Typically the luminance value is a blend of red, green, and blue in proportion to the sensitivity of the human eye, called by convention xe2x80x9cYxe2x80x9d. The chrominance requires two values, which, with the xe2x80x9cYxe2x80x9d value, define the three dimensions of color. A common representation of chrominance includes xe2x80x9cUxe2x80x9d, which is luminance minus red, and xe2x80x9cVxe2x80x9d, which is luminance minus blue. xe2x80x9cUxe2x80x9d and xe2x80x9cVxe2x80x9d are thus color tints layered on the monochrome xe2x80x9cYxe2x80x9d record. The human eye is much more sensitive to detail in the xe2x80x9cYxe2x80x9d channel, and thus xe2x80x9cUxe2x80x9d and xe2x80x9cVxe2x80x9d can tolerate less detail and more grain.
Color space conversion is a related art. Color space conversion maps an input suite of color channels into an output suite in a different color space or with different image colors. In a normal 3 dimensional color space conversion algorithm, three measurements at each image pixel, corresponding to the values at each pixel in each of the three sensed color channels, pass into the algorithm. That algorithm mathematically maps through a function to provide three measurements, or colors corresponding to that image pixel, out of the algorithm. For example, in digital development with prior art conventional color, film, as will be explained in more detail below, the xe2x80x9cfrontxe2x80x9d and xe2x80x9cbackxe2x80x9d images containing blue and red are subtracted from the xe2x80x9cthroughxe2x80x9d image containing red, green, and blue, to yield just the green. A small portion of this green image is subtracted from the xe2x80x9cfrontxe2x80x9d image to yield the blue image, and the xe2x80x9cbackxe2x80x9d image is mapped directly to red. In this color space conversion, for each pixel front, back, and through measurements from the front, back, and through channels pass in and red, green, and blue pass out. Color space conversion can be employed with any suite of channels to map from one set of colors to another, for example, to map grays to blues. Although some color space conversions can be defined by equations, in the general case a lookup table can be employed to give any arbitrary conversion.
A further related technology is direct digital development of images. This is a method of digitizing color film during development. The developing negative is scanned using infrared so as not to fog the developing film. Color is derived from a silver image during development by taking advantage of the milkish opacity of unfixed silver halide to separate the 3 layers optically. Viewed from the top during development, the top layer is seen clearly, while the lower layers are substantially occluded by the milkish opacity of the top layer. Viewed from the rear during development, the back layer is seen, while the other layers are mostly occluded. Finally viewed by transmitted light, the fraction of light that does penetrate all three layers is modulated by all, and so contains all 3. If the exposures of front, back, and through were mapped directly to yellow, cyan, and magenta dyes, a pastelized color image would result. However in digital development these three scans, front, back, and through, are processed digitally using color space conversion as explained above to recover full color.
The invention can also be practiced with layered sensors other than silver halide. A specific embodiment will be given for solid state sensing elements. Virtually all electronic imaging today uses silicon solid state sensors. When a photon strikes a silicon semiconductor, the photon knocks an electron from an atom, producing a hole-electron pair that allows a unit of charge to flow. Usually this charge is transferred to a small capacitor representing one picture element, or pixel, and this held charge is shifted in sequence with the charge from other pixels in a CCD, or Charge Coupled Device shift register, into an amplifier. Thus a xe2x80x9cCCDxe2x80x9d is a specific arrangement commonly used to read information from an array of solid state sensing elements.
A silicon solid state sensing element itself is sensitive to all visible colors. A full color image may be sensed by splitting light into three colored images with dichroic mirrors, and sensing each image with spatially separate sensing arrays. Such 3 chip cameras are very expensive and bulky, are generally not sensitive in low light because of light loss, and require expensive optics to project a virtual image of the lens aperture deep through the prisms.
An alternate and more common approach to color attenuates light with colored filters, exposing each individual sensing element to light from which at least one color has been removed. Some cameras designed for still photography use red, green, and blue filters laid in a matrix, such as the Bayer matrix used by Kodak cameras that place green over half the sensors in a square matrix to create a green checkerboard interlaced with a quarter of the pixels under red filters and a quarter under blue filters. Some cameras designed for video employ cyan, magenta, yellow, and green filters laid in matrix allowing easy conversion to YUV as the signal is read sequentially from the chip. Other colors and arrangements are used also. All such single chip cameras suffer from several problems. First, light removed by the filters is lost to the sensors. A particular single chip camera is rated ASA 100 with the colored filters in place. With the filters removed in a black and white version of the same camera, the rated speed jumps 4 times to ASA 400. Second, the colored matrix itself interacts with image detail to create colored moire artifacts common in single chip digital cameras. Typical manifestations of these moire artifacts include one red eye and one blue eye, or a distant building with randomly red and blue windows. Third, the color matrix reduces the effective resolution of the sensor array, and attempts to reduce colored moire artifacts by blurring with an optical antialiasing filter reduce the effective resolution.
The prior art has always believed that full color required sensors to operate in complementary color groups. The groups could include red, green; and blue, or cyan, green, magenta and yellow, but at least one color needed to be removed from each color sensor to complement some other sensor. This belief precluded the layering of a solid state sensor in which color response was incorporated with subtractive interlayer filters rather than variations in the color sensitivity of the layers themselves as in film, because it would have been impossible to remove any color for the top layer and still have it reappear for sensing at a lower layer.
A color image sensor responsive to light from an exposing light source which includes a first sensitive layer with a first spectral sensitivity. The color image sensor also includes at least one second sensitive layer with a second spectral sensitivity different from the first spectral sensitivity. The second spectral sensitivity is offset from the first sensitive layer in a direction perpendicular to the plane of the first sensitive layer. The first spectral sensitivity substantially matches the spectral sensitivity of the human eye.